Fix stale print progress telemetry

[ashimaryal25]

What changed

• Derive print percentage from layer / totalLayer when Creality reports a stale zero percentage while layers are advancing.
• Keep valid nonzero printer-reported percentages authoritative.
• Stop embedding the percentage inside displayJob, preventing the printer card from rendering it twice.
• Add regression coverage for the observed layer 9/18 with printProgress: 0 telemetry.

Root cause

The dashboard trusted printProgress unconditionally even though Creality firmware can leave that field at zero while continuing to update layer counters. Separately, both the backend job label and frontend progress row formatted the percentage, producing duplicated values.

Impact

A print at layer 9 of 18 now displays 50% consistently in Active Jobs and on the printer card, and the card shows the percentage only once.

Validation

• npm run check
• 37 unit tests
• Full simulator integration flow: upload, Auto-Print, pause, resume, cancel

[ashimaryal25]

Superseded by #3, which includes this progress fix plus the discovery, bed-safety, offline-state, and multi-file upload fixes.